Mechanism of Action
Doxycycline inhibits bacterial protein synthesis by binding to the 30S ribosomal subunit. Doxycycline has bacteriostatic activity against a broad range of Gram-positive and Gram-negative bacteria. Cross resistance with other tetracyclines is common. Doxycycline has been shown to be active against most isolates of the following microorganisms, both
in vitro and in clinical infections as described in the
INDICATIONS AND USAGE section of the package insert for doxycycline tablets.
Gram-Negative Bacteria
Acinetobacter
species
Bartonella bacilliformis
Brucella species
Calymmatobacterium granulomatis
Campylobacter fetus
Enterobacter aerogenes
Escherichia coli
Francisella tularensis
Haemophilus ducreyi
Haemophilus influenzae
Klebsiella species
Neisseria gonorrhoeae
Shigella species
Vibrio cholerae
Yersinia pestis
Gram-Positive Bacteria
Bacillus anthracis
Streptococcus pneumoniae
Anaerobic Bacteria
Clostridium species
Fusobacterium fusiforme
Propionibacterium acnes
Other Bacteria
Nocardiae and other
Actinomyces species
Borrelia recurrentis
Chlamydophila psittaci
Chlamydia trachomatis
Mycoplasma pneumoniae
Rickettsiae
Treponema pallidum
Treponema pertenue
Ureaplasma urealyticum
Parasites
Balantidium coli
Entamoeba species
Susceptibility Testing Methods
When available, the clinical microbiology laboratory should provide the results of
in vitro susceptibility test results for antimicrobial drugs used in resident hospitals to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting the most effective antimicrobial.
Dilution techniques
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized test method (broth and/or agar).
1,2,4 The MIC values should be interpreted according to criteria provided in
Table 1.
Diffusion techniques
Quantitative methods that require measurement of zone diameters can also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. The zone size provides an estimate of the susceptibility of bacteria to antimicrobial compounds.
The zone size should be determined using a standardized test method.
1,3,4 This procedure uses paper disks impregnated with 30 mcg doxycycline to test the susceptibility of microorganisms to doxycycline. The disk diffusion interpretive criteria are provided in
Table 1.
Anaerobic Techniques
For anaerobic bacteria, the susceptibility to doxycycline can be determined by a standardized test method.
5 The MIC values obtained should be interpreted according to the criteria provided in
Table 1.
Table 1: Susceptibility Test Interpretive Criteria for Doxycycline and TetracyclineBacteria Organisms susceptible to tetracycline are also considered susceptible to doxycycline. However, some organisms that are intermediate or resistant to tetracycline may be susceptible to doxycycline. | Minimal Inhibitory Concentration (mcg per mL) | Zone Diameter (mm) | Agar Dilution (mcg per mL) |
Bacteria
| S | I | R | S | I | R | S | I | R |
Acinetobacter spp.
Doxycycline
Tetracycline
| ≤4
≤4
| 8
8
| ≥16
≥16
| ≥13
≥15
| 10-12
12-14
| ≤9
≤11
| -
-
| -
-
| -
-
|
Anaerobes
Tetracycline
| -
| -
| -
| -
| -
| -
| ≤4
| 8
| ≥16
|
Bacillus anthracis The current absence of resistance isolates precludes defining any results other than "Susceptible". If isolates yielding MIC results other than susceptible, they should be submitted to a reference laboratory for further testing. Doxycycline
Tetracycline
| ≤1
≤1
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
|
Brucella species
Doxycycline
Tetracycline
| ≤1
≤1
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
|
Enterobacteriaceae
Doxycycline
Tetracycline
| ≤4
≤4
| 8
8
| ≥16
≥16
| ≥14
≥15
| 11-13
12-14
| ≤10
≤11
| -
-
| -
-
| -
-
|
Franciscella tularensis Doxycycline
Tetracycline
| ≤4
≤4
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
|
Haemophilus influenzae
Tetracycline
| ≤2
| 4
| ≥8
| ≥29
| 26-28
| ≤25
| -
| -
| -
|
Mycoplasma pneumoniae Tetracycline
| -
| -
| -
| -
| -
| -
| ≤2
| -
| -
|
Neisseria gonorrhoeae Gonococci with 30 mcg tetracycline disk zone diameters of less than 19 mm usually indicate a plasmid-mediated tetracycline resistant Neisseria gonorrhoeae isolate. Resistance in these strains should be confirmed by a dilution test (MIC ≥ 16 mcg per mL). Tetracycline
| -
| -
| -
| ≥38
| 31-37
| ≤30
| ≤0.25
| 0.5-1
| ≥2
|
Norcardiae and other aerobic
Actinomyces species
Doxycycline
|
≤1
|
2-4
|
≥8
|
-
|
-
|
-
|
-
|
-
|
-
|
Streptococcus pneumoniae
Tetracycline
| ≤2
| 4
| ≥8
| ≥23
| 19-22
| ≤18
| -
| -
| -
|
Vibrio cholerae
Doxycycline
Tetracycline
| ≤4
≤4
| 8
8
| ≥16
≥16
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
|
Yersinia pestis
Doxycycline
Tetracycline
| ≤4
≤4
| 8
8
| ≥16
≥16
| -
-
| -
-
| -
-
| -
-
| -
-
| -
-
|
Ureaplasma urealyticum
Tetracycline
| -
| -
| -
| -
| -
| -
| ≤1
| -
| ≥2
|
A report of
Susceptible (S) indicates that the antimicrobial is likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations at the infection site necessary to inhibit growth of the pathogen. A report of
Intermediate (I) indicates that the result should be considered equivocal, and, if the bacteria is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug product is physiologically concentrated or in situations where high dosage of drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of
Resistant (R) indicates that the antimicrobial is not likely to inhibit growth of the pathogen if the antimicrobial compound reaches the concentrations usually achievable at the infection site; other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the use of laboratory controls to monitor and ensure the accuracy and precision of the supplies and reagents used in the assay, and the techniques of the individuals performing the test.
1,2,3,4,5,6,7
Standard doxycycline and tetracycline powders should provide the following range of MIC values noted in
Table 2. For the diffusion technique using the 30 mcg doxycycline disk the criteria noted in
Table 2 should be achieved.
Table 2: Acceptable Quality Control Ranges for Susceptibility Testing for Doxycycline and TetracyclineQC Strain | Minimal Inhibitory Concentration (mcg per mL) | Zone Diameter (mm) | Agar Dilution (mcg per mL) |
Enterococcus faecalis ATCC 29212
Doxycycline
Tetracycline
| 2 – 8
8 – 32
| -
-
| -
-
|
Escherichia coli ATCC 25922
Doxycycline
Tetracycline
| 0.5 – 2
0.5 – 2
| 18 – 24
18 – 25
| -
-
|
Haemophilus influenzae ATCC 49247
Tetracycline
| 4 – 32
| 14 – 22
| -
|
Neisseria gonorrhoeae ATCC 49226
Tetracycline
| -
| 30 – 42
| 0.25 – 1
|
Staphylococcus aureus ATCC 25923
Doxycycline
Tetracycline
| -
-
| 23 – 29
24 – 30
| -
-
|
Staphylococcus aureus ATCC 29213
Doxycycline
Tetracycline
| 0.12 – 0.5
0.12 – 1
| -
-
| -
-
|
Streptococcus pneumoniae ATCC 49619
Doxycycline
Tetracycline
| 0.015 – 0.12
0.06 – 0.5
| 25 – 34
27 – 31
| -
-
|
Bacteroides fragilis ATCC 25285
Tetracycline
| -
| -
| 0.125 – 0.5
|
Bacteroides thetaiotaomicron ATCC 29741
Tetracycline
| -
| -
| 8 – 32
|
Mycoplasma pneumoniae ATCC 29342
Tetracycline
| 0.06 – 0.5
| -
| 0.06 – 0.5
|
Ureaplasma urealyticum ATCC 33175
Tetracycline
| -
| -
| ≥8
|